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Word Mark Details

E-mail From: Van Snyder < van.snyder@jpl.nasa.gov >
To: LaFarr Stuart < lafarr@zyvra.org >
Date: Sun, Jun 17, 2012 


LaFarr Stuart wrote:
> Stan is right.
>
> After I sent the email I was looking for more detail on the / command
> and found Wiki has a pretty good write up about the 1401 and near the
> bottom there is a reference link to a PDF file which seems to be the
> latest "Brownie book" with a few penned in details. Assuming it is all
> correct there are about a half dozen instructions that don't have to
> be followed with a character with a word mark set.
>
> Incidentally, and I don't remember how I found it out; but a quick and
> easy way to no-op an instruction is to simply clear its' word mark.
> For this to work the proceeding instruction needs to be one that
> requires the following word mark. I used this "trick" in lots of
> programs I wrote, and made sure RCA's simulators & emulators made it
> work.
>
> What I have just said in essence is: 1401 instructions can be as short
> as one character and any amount longer. If the instruction is too long
> only the first 7 or 8 characters will be used. I think everybody knows
> about "chaining" where something like the add instruction can be only
> 1 character long. A two and three character add would be very tricky.

A three- or five-character instruction would likely cause a check stop 
with an invalid address. An eight- (or more) character add is probably 
OK -- the D modifier would probably be ignored. In a two- or 
five-character add, I don't know whether the first character of the 
A-STAR or B-STAR would be clobbered.

Clearing the word mark in an instruction to turn it off could have 
surprising consequences. An instruction with a D-modifier uses the last 
character fetched, not the eighth one. The safest thing is to precede 
the instruction to be switched with a NOP. An initially-off branch is 
more easily set up as a five-character branch with a group mark for a 
D-modifier, since nothing ever sets the switch that would inquire (or 
maybe there's not even a latch for it). Setting a word mark under the 
group mark turns on the branch.

I'm interested to know which instructions in addition to seven-character 
set word mark, seven-character clear storage and branch, and 
five-character branch with a blank D-modifier can be executed without a 
word mark in the next character.

Finally, you ought to replace vsnyder@mls.jpl.nasa.gov with 
van.snyder@jpl.nasa.gov or van.snyder@sbcglobal.net. I don't know how 
much longer the MLS address will last.

Van

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